Signal modulators are utilized in a number of applications, for instance as part of the conversion from analogue signals to digital signals. For example sigma-delta (ΣΔ) modulators (SDM's) are a type of signal modulator that may be used to convert an input analogue signal into a pulse-density-modulated (PDM) signal comprising a series of digital ones and zeros at a fixed sample rate where the relative density of ones and zeros corresponds to the analogue signal's amplitude. However the fixed sampling frequency inevitably introduces quantisation noise into the signal, and SDMs usually comprise also at least one functional operational amplifier.
Time-encoding modulators (TEMs) are modulators that encode input signals into a time-encoded data stream. One particular form of time-encoding is pulse-width modulation (PWM). In a PWM signal, an input value is encoded by the duration of a given output signal level, e.g. the duration or width of pulse of a first signal level, compared to the duration of any period(s) of any other signal level(s) in a cycle period. For a conventional two-level PWM signal, the input signal value may be encoded by the duty cycle of a pulse of a first signal level within the cycle period, i.e. the proportion of the cycle period spent at the first output signal level. Time-encoding modulators may encode an input signal into a PWM signal by comparing the input signal with a periodic reference signal, such as a triangular waveform to encode the input signal by the duration of pulses in the output signal. However this requires circuitry to generate an appropriately accurate periodic reference signal and/or operational amplifier (op-amp) circuitry.
In general there is a desire for smaller and/or lower power modulators that can be used, for example, as part of a signal converter such as an analogue-to-digital converter (ADC).
In particular, in some applications a modulator may be used, e.g. as part of an ADC, in a signal path that may be intended to operate continuously to be able to receive data at any time, but where data of interest may only be received periodically. For example, some devices, such as mobile phones, voice assistants, personal assistants etc., may have the functionality to be able to respond to voice commands. Such devices may thus have a microphone for receiving acoustic signals, an ADC for converting the received audio into a digital signal and a speech recognition processor for processing the digital audio to identify spoken commands. In some instances the voice control functionality may be enabled by a user physically interacting with some user interface of the device, and thus the relevant signal path including the ADC and speech processor may only be enabled in response to such user input. However for a convenient hands-free user experience it would be desirable for a user to be able to speak commands directly without first having to prime the device by pressing a button for example. Such functionality requires the relevant signal path to be able to receive and identify suitable spoken commands at any time. However having the microphone, ADC and speech processor all continually powered and active would involve a reasonably significant and continuous power consumption and, especially for battery powered devices, power consumption is important. Similar considerations also apply to microphones and circuitry that may be arranged to receive data transmitted at ultrasonic frequencies for machine-to-machine communication.
It is therefore known that some elements, such as a speech processor, may be disabled and substantially unpowered unless and until it is determined that there is significant signal content in the output of the microphone and, in some implementations, that the significant activity corresponds to a particular signal of interest, e.g. speech or an ultrasonic data signal. To provide this functionality, the microphone and ADC may be powered, with some minimal processing of the resultant digital audio signal to determine whether there is any significant activity. If significant activity is detected, other processing elements may be enabled, possibly in a series of stages, e.g. to verify that the activity corresponds to speech and/or corresponds to a defined command word or phrase and/or corresponds to a particular user. In this way only the microphone and ADC, and some minimal activity detector, are continuously powered and active. It would therefore be desirable for the ADC to be operable with a relatively low power consumption for such ‘always-on’ operation.
Once relevant activity is confirmed various additional processing modules may be fully enabled to process the signals, e.g. to apply speech processing, and in some applications it may be desirable for the signal path to be relatively high quality so as to reduce errors in the processing. Additionally or alternatively the microphone and ADC might also, at other times, be used for other purposes. For example the same microphone and ADC may also be used, for example for voice calls or recording audio, so as to avoid having to provide an entirely separate audio path. For such other uses a high quality audio signal may be desirable and therefore it may therefore be desirable that the ADC be operable with relatively high quality.